1. Field of the invention
This invention relates to optical detection and relates to a photodetector based on buried junctions and its method of manufacture, an optical sensor, systems for color imaging, artificial vision, color measurement and spectral measurement and a method of spectral measurement.
It has applications particularly in the field of color imaging, for video cameras producing color images. These cameras can be intended for the public at large, such as video cameras or visual conference cameras or for professional use, such as, for example, those used for audio-visual production or for medical imaging.
The invention also has applications in the field of artificial vision, notably for artificial retinas for the detection of colors.
A third field of application of the invention is concerned with the measurement of colors, for systems such as, for example, scanners or devices for matching colors for color printing.
The fourth field of application, spectral measurement, relates to the measurement of the absorption, emission or reflection spectrum of a solution, a solid or a colored gas or of a smoke, as well as the measurement of the spectral composition of light which is a determinant for the measurement of properties of a physical, chemical or biological phenomenon. Hence this field of application covers colorimetry which consists of determining the principal or dominant wavelength of light from an object or a colored substance in such a way as to obtain its summary color. Colorimetry can be notably used in the chemical field (measurement of absorbance, pH measurement of a substance, or the indirect measurement through injection of a coloring agent), in the biomedical field (biological or biochemical) and in the environmental field (gas measurement, pollution detection, measurement of concentration of elements in a solution). This field of application also covers spectrocolorimetry consisting of determining a color spectrum by spectral dispersion.
By "photodetector" one understands a device that responds to the presence of optical radiation producing an electromotive force or a variation in electrical resistance which is independent of the spatial distribution of the optical radiation. By "optical sensor" one understands, more generally an input device for a measurement, detection, analysis, recording, reproduction or transmission chain, that is sensitive to optical radiation due to the use of a photoelectric effect. An optical sensor can therefore include a plurality of photodetectors.
2. Description of the Prior Art
It is known to use optical luminous intensity detectors, such as photodiodes and phototransistors. These components give an electrical signal which is a function of the intensity of incident light, but do not supply any information concerning the spectral composition of this light.
So as to detect three chromatic components corresponding to the color of an object or of light, a system with three CCD sensors has been suggested. The three sensors are each fitted with an optical filter arranged in front of the sensor assembly. The three filters are generally associated with the three complementary colors, blue, green and red.
This system however has the disadvantage of requiring expensive, specific and precise optics.
Furthermore, since each sensor includes an array of photodetectors, three photodetectors belonging respectively to the three sensors are set to work for each pixel of a color image. This optical acquisition system therefore occupies a photodetector surface area three times that of an acquisition system without color whilst at the same time the precision of the measurements of the distance between pixels is reduced.
An optical acquisition system for color images is also known that includes a single CCD sensor. The sensor comprises a plurality of photodetectors, on which optical filters are directly deposited. Generally, blue, green and red optical filters are alternated in an array of photodetectors.
A disadvantage of this optical system is that it necessitates recourse to specific technology for the deposition of the optical filters. Furthermore, this second system has the same disadvantages as the first in relation to the surface area necessary for each pixel and the precision of the measurements of distance between pixels. Finally the array of photodetectors has a paving that lacks geometrical symmetry.
In relation to spectral measurement, a dispersive unit or a set of optical filters, arranged in front of a single detector or a linear detector array is generally used.
The system with a single detector has the disadvantages of requiring mechanical displacement means, difficult to produce or expensive and not allowing measurements to be made in real time.
As for the system with a linear array of detectors, it has the disadvantage of providing a measurement accuracy that depends on the precision of the positioning of the linear array and of the calibration of the system.
Furthermore, the two systems have the common disadvantages of producing spectral measurements which are greatly dependent on the dispersive unit, and very sensitive to the operating conditions of the apparatus, notably through fluctuations of the spectrum and of the intensity of the light source.
A method and a system for determining the wavelength of light incident on a photosensitive semiconductor structure has been proposed in the patent U.S. Pat. No. 4,749,851.
A disadvantage of the system disclosed is that the measured wavelength is prone to variations in relation to measurement voltages. Furthermore, this system makes it necessary to develop a structure with a high doping level.